Varactors are commonly used in RF circuits for tuning oscillators, filters and amplifiers.
One problem with varactors is that their capacitance/voltage characteristics are typically very non-linear as shown in FIG. 1A, which illustrates a typical metal oxide semiconductor varactor (MOSvar) capacitance/voltage characteristic. The non-linear feature of the MOSvar is emphasized by FIG. 1B which shows the first derivative dC/dV of the curve of FIG. 1A.
One device allowing a capacitance/voltage characteristic having an acceptable tuning range and a more linear range to be obtained is a hyper-abrupt varactor. However, the implementation of a hyper-abrupt varactor requires extra processing during manufacture, which is expensive.
An alternative method of overcoming the non-linearity of a varactor is to use digital techniques to switch in capacitors so as to tune over the required range. However, this solution is complex, can be physically large, and may be too slow.
In a co-pending patent application filed by the present applicants and having the same priority date as the present application, priority being claimed from GB 0327284.6, there is disclosed a circuit arrangement having a variable capacitance for a tuning circuit, the arrangement comprising a plurality of variable capacitance elements, preferably varactors, connected in parallel. Coupled to the varactors are control means for electronically controlling the capacitances of the varactors, the control means having a control range (e.g. a control voltage range) over which they cause the capacitance of the circuit arrangement to vary. The control means and the varactors are configured such that at least one of the variable capacitance elements exhibits variation of its capacitance in response to the control means over only a portion of the control range. Such an arrangement can be used to provide a more linear capacitance response with respect to a control variable, or a response which more closely follows a required non-linear characteristic, than is generally obtainable with a single varactor.
Indeed, the preferred arrangement provides a different offset for each varactor in a group of varactors so that variation in the overall capacitance of the arrangement is caused by variation of the capacitances of the varactors in respective consecutive voltage ranges of a common control voltage. In some implementations of the circuit arrangement, e.g. in a voltage controlled oscillator (VCO), it is necessary to add a fixed value capacitor in series with the parallel-connected varactors, e.g. to set the center frequency of a VCO. The addition of the series capacitor can have the effect of modifying the capacitive response of the arrangement to the extent that the required approximately linear capacitance versus control variable response, or linear frequency versus control variable response is no longer obtained. This effect is of particular significance when the capacitance of the series capacitor is smaller than the capacitance of the varactors, and can result in the capacitance characteristic being comparatively steep over an initial portion of the control range, e.g. when one of the varactors is at a mid-point of its variable capacitance range and the others are each set at the low end of their capacitance ranges. In other words, the series capacitor can result in variation of the capacitance of a first varactor producing a large dC/dV value which decreases as the other varactors are brought into operation. This is illustrated in FIG. 8B.